The present invention concerns polycarbonate and more particularly polycarbonate resin suitable for making containers.
A thermoplastic molding composition that contains a polycarbonate resin having specific extensional rheological properties is disclosed. The composition is particularly suitable for preparing containers.
Containers made of polycarbonate are known in principle. These containers are produced from compositions (also known as compounds) containing polycarbonate and conventional functional additives, for example. These compositions contain the polymer (polycarbonate) and functional additives are also known as thermoplastic molding compositions. Examples of the conventional additives, include stabilizers, processing aids and others. The containers made of polycarbonate may also include other components, such as rubber seals or handles made from metal or other materials, for example.
Containers made of polycarbonate display numerous advantageous properties, such as e.g. high transparency, good mechanical properties, high resistance to environmental influences and long service life, together with low weight and simple, inexpensive manufacturability.
Containers made of polycarbonate may be produced by the extrusion blow molding process or by the injection blow molding process, for example.
In extrusion blow molding the polycarbonate is generally melted with a single-screw extruder and molded through a die to form a free-standing parison. The parison usually hangs down from the die. A blowing mold is then placed around the parison, squeezing together the lower end of the parison. Inside the mold the parison is then blown up so that the parison attains the desired shape. After a cooling period the mold is opened and the container (hollow article) may be removed.
Extrusion blow molding is disclosed for example in Brinkschroder, F. J.: xe2x80x9cPolycarbonatexe2x80x9d in Becker, Braun, Kunststoff-Handbuch, Volume 3/1, Polycarbonate, Polyacetale, Polyester, Celluloseester, Carl Hanser Verlag Munich, Vienna 1992, pages 257 to 264).
The injection blow molding process is a combination of injection molding and blow molding.
Injection blow molding proceeds in three stages:
1. Injection molding of the parison in the plastic temperature range of the polycarbonate
2. Blowing of the parison in the thermoplastic range of the polycarbonate (the core of the injection mold also acts as the blowing mandrel)
3. Stripping of the hollow article and optional cooling of the blowing mandrel with air
Injection blow molding is disclosed for example in Anders, S., Kaminski, A., Kappenstein, R., xe2x80x9cPolycarbonatexe2x80x9d in Becker,/Braun, Kunststoff-Handbuch, Volume 3/1, Polycarbonate, Polyacetale, Polyester, Celluloseester, Carl Hanser Verlag Munich, Vienna 1992, pages 223 to 225.
The disadvantage of the containers made of polycarbonate known from the prior art is that they do not meet certain requirements that are important for the use of the containers in practice.
Thus for example the known polycarbonate containers may burst under severe mechanical loading. This may occur for example if a container filled with liquid is dropped to the ground from a great height, for example from the loading platform of a lorry on which the container is being transported.
The reason for this mechanical failure is typically an uneven wall thickness of the container.
The uneven wall thickness of containers known from the prior art arises during their production, since the polycarbonate melt gives rise to uneven wall thicknesses during processing by the extrusion blow molding process or by the injection blow molding process.
The mechanical strength of containers having an uneven wall thickness may naturally be increased by using much more polycarbonate per container, such that the cross-section of the wall becomes much thicker. This has the disadvantage of increasing the material consumption, however, giving rise among other things to higher cost.
The object of the present invention is therefore to provide a polycarbonate that allows the production of containers having as homogeneous a wall thickness as possible.